1. Field of the Invention
This invention relates to photographic additive aqueous dispersions having a neglectable amount of crystallization therein and to a method of preparing such dispersions.
2. Description of related Art
Photographic additives are usually incorporated into photographic systems as a colloidal emulsion, usually called a dispersion in the photographic art. The additives are very often water insoluble or substantially water insoluble and this characteristic makes the preparation of the dispersion difficult especially where small particles are desired. To prepare these dispersions, the photographic additives are added to a high boiling water immiscible solvent, generally called a permanent solvent. At times, a low boiling solvent or a water miscible solvent (generally called an auxiliary solvent) is added to promote the solubility of the photographic additives. The thus formed mixture is heated to form a solution. This solution is mixed under high shear, together with an aqueous gelatin solution, generally containing a surfactant at elevated temperatures in order to break the organic phase (oil phase) into sub-micron droplets dispersed in the continuous aqueous phase. When an auxiliary solvent is employed, it is removed from the dispersion prior to the employment of the dispersion in the preparation of a photographic element. Regardless of whether or not an auxiliary solvent is employed, a common problem regarding dispersions of water insoluble photographic chemicals is that they are frequently unstable. One result is the formation of crystals of the chemicals in the dispersion. These crystals can interfere with the functioning of the dispersion, its coatability and it optical properties. It is therefore desirable to suppress crystal formation in photographic dispersions.
U.S. Pat. No. 3,860,425 describes the use of non-ionic surfactants containing polyoxyethylene and polyoxypropylene units as dispersion aids for the suppression of crystal formation.
East German Patent DD 277,343A discloses the use of polysiloxane/polyether copolymer surfactants utilized in the preparation of photographic dispersions to decrease the amount of coarse dispersed particles of high boiling organic solvent. The invention disclosed in this East German patent eliminates dispersed particles greater than 0.2 .mu.m, thereby improving the transparency of layers produced from the photographic dispersions.